With AC, we are dealing with a Sine Wave, which means that the Current is not in a steady state as the voltage swings, typically at 60 times a second here in the states.

That means that the old "0.707" multiplier applies.

So the same AC voltage will read a lower current than the DC voltage.

or, DC voltage times .707 would be the AC voltage. Or Current.

Again, the Current reading is not necessarily proof that the bulbs are good or bad, but it helps. If the Current had been inordinately high for the bulb spec, it would be a proof. But even if it is at or under spec, the Current reading says nothing about other parameters such as filament metallurgy, etc.

Martin, Your #47 light problem is due to over voltage, no doubt. Connect your volt meter to the pilot light when the receiver is off. Then turn on the power. If your meter is fast enough try to catch the voltage at start-up to see if it exceeds the voltage rating of the #47 lamp. If it does you have part of your answer and are dealing with a surge voltage. If you have a surge start-up voltage, start swapping tubes. I suspect a filament of a tube is not responding appropriately. Look at the tube that the #47 is in parallel with it. If that doesn't help you can put in a an NGC thermistor in circuit to dampen the start-up current.

Rather than futzing round with series resistors, I would recommend replacing the 6.3v lamp (usually rated at 6-8 volts) with the equivalent lamp designed for 12.6v, usually rated at 12-14 volts. Nearly any style of 6.3v pilot lamp has a 12.6v equivalent. Running at half voltage, they still put out plenty of light and seem to last for ever.

I had the same problem with my Collins 75A-4 receiver. I replaced all the lamps with 12v ones, and I haven't replaced a pilot lamp in over 10 years. Another advantage of the higher voltage bulbs is that the original 6.3 volt ones gave out a bright, harsh white light that was especially annoying if I decided to operate the receiver with the shack lights turned down low. The 12v replacements give out a warm, pleasant amber glow that is much more comfortable.

Martin, Your #47 light problem is due to over voltage, no doubt. Connect your volt meter to the pilot light when the receiver is off. Then turn on the power. If your meter is fast enough try to catch the voltage at start-up to see if it exceeds the voltage rating of the #47 lamp. If it does you have part of your answer and are dealing with a surge voltage. If you have a surge start-up voltage, start swapping tubes. I suspect a filament of a tube is not responding appropriately. Look at the tube that the #47 is in parallel with it. If that doesn't help you can put in a an NGC thermistor in circuit to dampen the start-up current.

I measured it this morning both with the VTVM and the DMM and got identical results: Across the filament of the tube that the pilot is connected to: initial voltage 6.8VAC, slowly dropping over the following 120 seconds and eventually settling on 6.6V. This is slightly puzzling because the last time I measured it (10 days ago prior to installing the 10-ohm series resistor on the pilot) it was exactly 6.3V after warmup. Two possibilities: (1) installing the series resistor on the pilot lowered the current load on the transformer sufficiently to cause the voltage increase, or (2) the supply voltage in my neighborhood varied enough to explain the difference (the measured house voltage this morning is a very high 126.5 volts on the DMM). I don't think (1) makes much sense so it is probably (2).

*However* aren't these bulbs (and tube filaments, for that matter) pretty tolerant of a small overvoltage? This radio has run hundreds of hours since I obtained it a year ago and the tubes are still all lighting. In some catalogues, #47s are rated to run 6-8V. Does 6.8V, settling down to 6.6, count as a large surge? Would it be a good idea to install a resistor in series with the entire heater circuit, to prolong the life of the tubes?

Meanwhile, 10 days later, the pilot lamp is still doing fine with its series 10-ohm resistor. I turn it on first thing in the morning then turn it off at night, so it has now been more than 100 hours.

Rather than futzing round with series resistors, I would recommend replacing the 6.3v lamp (usually rated at 6-8 volts) with the equivalent lamp designed for 12.6v, usually rated at 12-14 volts. Nearly any style of 6.3v pilot lamp has a 12.6v equivalent. Running at half voltage, they still put out plenty of light and seem to last for ever.

I had the same problem with my Collins 75A-4 receiver. I replaced all the lamps with 12v ones, and I haven't replaced a pilot lamp in over 10 years. Another advantage of the higher voltage bulbs is that the original 6.3 volt ones gave out a bright, harsh white light that was especially annoying if I decided to operate the receiver with the shack lights turned down low. The 12v replacements give out a warm, pleasant amber glow that is much more comfortable.

Indeed, my dial, too, looks nicer after inserting the resistor in the circuit. It is a yellow glow rather than white.

The pilot light burned out! (Not very long after my previous post.) So even at 5 volts with the series resistor, a brand new "Linrose" branded bulb only lasted about 100 hours.

I have now replaced the blown Linrose with a functioning, 60-year-old GE brand bulb pulled from the junkbox, but I am leaving the 10-ohm resistor in the circuit. After warmup, the measured voltage across the GE bulb is about 5 volts. The experiment continues.

Yeah, I agree that the #47 should handle 6.8V. Indeed a puzzling problem. Rather than using a meter how about an Oscope on the light bulb socket and repeating a cold start. You're going to have to rely on the speed of your eye to capture the turn-on unless you have a storage scope.

Rather than using a meter how about an Oscope on the light bulb socket and repeating a cold start.<<

My Oscope is a work in progress. It's a 60-year-old Eico 460, awaiting rebuild. It "works" (there is a trace, and it displays a sine wave or square wave if fed one) but is uncalibrated and there are all sorts of bizarre modifications inside that don't seem to make sense and need to be reversed out ... carefully, given the voltages in there. I don't really want to use it until the repairs have been done!

What's the reasoning behind using a diode? Obviously you'd get a voltage drop, and the bulb would be fed halfwave DC.

As of now I am running the radio with the "old stock" GE bulb (and the resistor left in the circuit) and will see how long it holds out before blowing. If it doesn't blow, that would be strong evidence that it's the brand of new bulbs I've been using that is the source of the problem.

Hi Martin,The diode is a crap shoot. I was thinking that using a diode would lower the "duty cycle" by rectifying out one half the sine wave. I don't know if you'd gain anything but it might be worth a try.

As far as the scope goes, numbers aren't critical at this point. If you can simply monitor the AC from cold start to steady state see if there's an excursion prior to steady state that might yield a clue. I'm wondering if the meter isn't fast enough to capture the startup voltage but the scope might capture it.

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